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IAUC 5887: 1993ad; CRAB NEBULA

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                                                  Circular No. 5887
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505

SUPERNOVA 1993ad IN IC 1501
     C. Pollas, Observatoire de la Cote d'Azur, reports his discovery
of a supernova of mag B = 18 on two films taken Nov. 7.87 and
8.88 UT.  The object is located at R.A. = 23h32m06s.75, Decl. =
-3o26'08".5 (equinox 1950.0), which is 13" east and 22".7 south of
the nucleus of the spiral host galaxy.  Nothing was found at this
position to mag 20 on films taken Oct. 20.88 and to mag 21 on Aug.
19.09.  A nearby star is at end figures 12s.89, 23'52".8.  E.
Cappellaro and M. Della Valle, European Southern Observatory,
report: "A flux-calibrated CCD spectrogram (range 380-680 nm,
resolution about 1 nm), obtained on Nov. 10.1 UT with the 3.6-m
reflector (+ EFOSC) at La Silla, shows this object to be a type-II
supernova at a very early stage.  Preliminary analysis reveals
emission lines of the Balmer series from H-alpha to H-epsilon
superimposed on a very blue continuum.  Other emission lines are
measured at 462, 476 (likely He II 468.6), and 590 nm.  All features
are narrow (FWHM about 1500 km/s).  The spectrum bears strong
similarity to that of SN 1983K ten days before maximum (Niemela et al.
1985, Ap.J. 289, 52).  Provisional photometry gives B = 16.3, B-V =
-0.3, V-R = 0.0.  The H-alpha emission of the galaxy gives a
redshift of z about 0.018."

     O. C. De Jager and H. I. Nel, Potchefstroom University, South
Africa; A. K. Harding, P. Sreekumar, and D. J. Thompson, Goddard
Space Flight Center, NASA; and P. F. Michelson and P. L. Nolan,
Stanford University, report:  "EGRET gamma-ray observations of the
Crab unpulsed component yielded the following flux levels (units of
10E-6 cmE-2 sE-1) for the energy range 0.07-1 GeV:  1991 Apr. 23-
May 30, 1.18 +/- 0.11; 1992 June 11-1993 May 24, 0.58 +/- 0.14.
The significance of the reduction in flux is 3.4 sigma.  The energy
spectrum of the range 1-30 GeV is much flatter, and no reduction in
flux is seen for this range.  The variability below 1 GeV (as
predicted by De Jager and Harding 1992, Ap.J. 396, 161) may be
associated with unstable electron acceleration at the pulsar wind shock
at a distance of 8" from the pulsar (the 'Crab wisps').  EGRET
observations of the Crab region continue between 1993 Dec. 1 and 13,
and we urge optical observations (e.g., CCD) of the central Crab
region.  The measurement of temporal broadening from radio pulsar
scintillation (cf. Gwinn et al. 1993, Ap.J. 410, 673) is also

1993 November 10               (5887)            Daniel W. E. Green

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